It is common in printers to have means to change the size of the paper gap between cooperating elements of the print mechanism, such cooperating print elements being either print hammers and a type carrier, such as an engraved band or drum, of high speed impact line printers or the print head, wheel or other impression forming elements and the platen of serial type printers. The reason for changing the gap is for printing on a medium, such as a paper web, having different thicknesses. In high speed printers, it is desirable to be able to adjust the gap in a manner such that distance between the print hammers, for example, and the paper is constant regardless of the thickness of the paper. Basically, gap adjustment to accommodate variations in paper thickness is a fine adjustment more or less precise. Another common form of gap adjustment is one in which the gap is merely opened or enlarged to the degree that the print medium, irrespective of thickness, can be conveniently installed or removed from the printer. Enlarging the gap to enable installation of the print medium is basically a gross or coarse change. Ordinarily, the fine and coarse gap adjustments were accomplished through separate mechanisms. As a consequence, the gap changing mechanisms have tended to be complex thereby adding to the cost of construction and operation of the printing machines.
For example, the cooperating print elements, such as print hammers and type carrier of a line printer, are conventionally mounted facing each other on separate support structures or frames which are relatively movable. The means for adjusting the paper gap between the cooperating print elements to accommodate paper of different thicknesses comprises cam means on a rotatable shaft manually rotated by a lever to cause relative movement of the frames. The means for opening the gap comprises a handle on the one frame, commonly called the swing frame, which is connected by a horizontal or vertical hinge structure to the other frame. An early example of this arrangement is shown in U.S. Pat. No. 3,155,032. Later examples of such gap changing mechanisms are found in U.S. Pat. Nos. 4,248,146; 4,773,772 and 4,932,797. Other examples of printers with separate types of gap changing mechanisms can be seen in U.S. Pat. Nos. 4,210,076 and 4,780,007. A printer having a single sector or worm gear for adjusting the gap between a print head and platen is shown in U.S. Pat. No. 4,420,269. IBM Technical Disclosure Bulletin, Vol. 24, No. 11B, dated Apr. 1981, pp 6158 et seq. shows a adjustment mechanism for a platen and a print head which uses a stepped block or inclined plane for making the adjustment. Neither shows a single mechanism capable of performing fine and coarse gap changes.